test(billing): add Slumber API spec and localhost env config

localhost_3000.env

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+# Server URL: 
+BASE_URL=http://localhost:3000

slumber.yml

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+name: monk-code-screen billing API
+profiles:
+  http://localhost:3000:
+    name: http://localhost:3000
+    data:
+      host: http://localhost:3000
+requests:
+  listCustomers:
+    name: List customers
+    method: GET
+    url: '{{ host }}/customers'
+  listCustomerInvoices:
+    name: List a customer's invoices
+    method: GET
+    url: '{{ host }}/customers/{{ id }}/invoices'
+  getHealth:
+    name: Liveness check
+    method: GET
+    url: '{{ host }}/health'
+  ingestCsv:
+    name: Ingest invoices from CSV
+    method: POST
+    url: '{{ host }}/ingest'
+    body: '"invoice_number,customer_name,customer_email,invoice_date,due_date,description,quantity,unit_price,amount,currency\nINV-1001,Acme Corp,billing@acme.com,2026-01-05,2026-02-04,Consulting services,10,150,1500,USD\n"'
+  getInvoice:
+    name: Get an invoice with its payments
+    method: GET
+    url: '{{ host }}/invoice/{id}'
+  updateInvoice:
+    name: Update an invoice
+    method: PUT
+    url: '{{ host }}/invoice/{id}'
+    body:
+      type: json
+      data:
+        amount: 0
+        currency: USD
+        description: ''
+        due_on: 2026-03-01
+        quantity: 1
+        status: open
+        unit_price: 0
+  createPayment:
+    name: Pay an invoice
+    method: POST
+    url: '{{ host }}/payment/{{ invoice_id }}'
+    body:
+      type: json
+      data:
+        amount: 1
+        currency: ''
+        idempotency_key: ''